It has been in vogue for construction machines having a swing structure such as hydraulic excavators to use their engines to drive a hydraulic pump and run a hydraulic motor by a hydraulic pressure delivered from the hydraulic pump so as to drive its swing structure, an inertial structure. However, in recent years, there have been proposed a hybrid type construction machines proposed in terms of ensuring improved engine fuel economy, reduced noise level, and reduced exhaust gas. These construction machines use two kinds of motors to drive the swing structure, a hydraulic motor as has been already used and an electric motor driven by electric energy supplied from a power storage device.
In a hybrid type construction machine that drives its swing structure in a combined manner, it is necessary to control the drive torque shared by the hydraulic and electric motors such that operators accustomed to operating conventional construction machines (conventional machines) that drive their swing structures with a hydraulic motor alone can manipulate the hybrid type construction machine with no sense of discomfort.
JP-2008-63888-A discloses, as a control means of a hybrid type construction machine of the above kind, a technique for calculating a torque instruction value to be supplied to the electric motor that drives the swing structure based on the differential pressure between two ports provided on the hydraulic motor for driving the swing structure, the two port being a hydraulic fluid inlet (incoming side) and a hydraulic fluid outlet (outgoing side). Here, the torque ratios between the hydraulic and electric motors at the time of acceleration of the swing and at the time of deceleration of the swing are defined by the differential pressure between the incoming and outgoing sides of the hydraulic motor as a parameter. This technique allows for successive and smooth driving and control of the swing structure, an inertial structure, and also permits efficient capture of braking energy into a power storage device as electric energy.